Cracking of liquid hydrocarbons



Nov. 6, 1928.

O. D. LUCAS ET AL CRACKING 0F LIQUID HYDROCARBONS Filed July 25, 1925Patented Nov. 6, 1 928.

UNITED STATES 1,690,416 PATENT OFFICE.

OWEN DAVID LUCAS AND ERNEST LAWSON LOMAX, OF WESTMINSTER, LONDON, ENG-LAND, ASSIGNORS TO V. L. GIL PROCESSES LIMITED, OF -WESTMINSTER,ENGLAND,

A BRITISH COMPANY.

CRACKING OF LIQUID HYDROCARBONS.

. Application filed July 23, 1925, Serial No. 45,525, and in GreatBritain December 9, 1924.

but is maintained at the full pressure and protected against loss ofheat. From this chamber the hydrocarbon passes through a suitable reliefvalve into an expansion chamher, after which it is fractionallycondensed. Under these conditions a conversion in one operation of theorder of 40 per cent may be obtained, but after the lighter oils areremoved the heavier portion may be returned through the process and afurther converk to supply the heat to the conversion chamber,

sion obtained.

We have found that the reason for the limitation of the percentage ofconverted hydrocarbons obtainable in a single opera tion is thereduction of the conversion rate b the increasing concentration of thelighteroils, produced by the cracking, in the resi-' dues. We havefurther found that the rate of cracking is effected by the difference intemperature between the incoming oil and the oil contained in thereaction chamber, that is to say, the larger the difference of 36.temperature the greater the velocity of the reaction, but this velocityagain diminishes as the concentration of the lighter spirit in theheavier oil increases. According to this invention the lighter 4ohydrocarbons or oils are removed from the conversion or cracking chamberin a continuous manner as they are formed, under conditions oftemperature and pressure which enable the cracking reaction to prowed inthe body of heavier oils which is therefore free from such proportion ofthe lighter oils already produced by the cracking operation as wouldmaterially reduce the efficiency of the reaction, In the preferredprocess the lighter oils, together with any permanent gases that areobtained, are drawn off in vapour form into a vapour pres sure orrefluxing chamber maintaining a temperature substantially below thetemper.-

ature of the conversion chamber, but sufliciently high to prevent, underthe pressure employed (that of the conversion chamber), the condensationof any oils below a predetermined density, while heavier oils which maybe carried over are condensed in the refluxing chamber and then passthrough a reflux cooler leading the oils condensed back into theconversion chamber for further treatment.

Fresh oil is admitted to the conversion chamber with or independently ofthe returned oils and the entering oil may be preheated to a temperaturesubstantially below the cracking temperature, in which case it may besprayed or allowed to drop upon the upper surface of the oils undergoingcracking; or it may be admitted, with or without the refluxed oils, tothe lower part of the conversion chamber to pass into the body of.

heavier oils, in which case the preheating may be carried up to thecracking temperature which then may require no external heating but maybe merely lagged to avoid loss.

In order that the said invention may be clearly understood and readilycarried into effect, the same will now be described more fully withreference to theaccompanying diagram, which illustrates the generalarrangement of a plant adapted to carry out the present process. Theindividual pieces of-apparatus employed are of known type and are notshown in a constructional form.

A is the oil feed tank from which the oil passes through the non-returnvalve a to the pressure pump a B is a pipe still in which theoil isheated. C is the cracking or conversion chamber'. D is the refluxingchamber on the top of the chamber C and E is' the reflux coolerconnected to the chamber D at one side and to the conversion cham-'light oils, N a gas separator, O, 0 light oil receivers and P agasholder.

The oil from the feed tank is pumped by the pressure pump a through theheat exchanger K and thence through the pipe still B which is heated bygas from the gas holder 1 or direct from the gas separating end of theplant or by oil from the fuel tank R. The oil passes from the pumpthrough the valve (1 to the heat exchanger K and from this through thevalve is to the pipe still, the still comprising the coil 11 connectedto the inlet pipe and the outlet pipe If. The heated oil passes throughthe pipe 6 to the conversion or cracking chamber C, which is preferablymerely lagged and is not separately heated, the pipe still heating theoil sufficiently for the re-action' in the conversion chamber. When theoil in the conversion chamber reaches the necessary temperature theprocess may be carried on by pumping the fresh oil 'into the chamberthrough the pipe 0 with valves 0 and 0, into the top of the chamber C sothat the oil drops on top of the heated body of oil in the chamber; orthe oil may continue to be passed through the body of oil in theconversion chamber, entering at the bottom.

It will be seen that there are two alternative methods of feeding thefresh oil to the conversion chamber C, one in which the oil is preheatedto a temperature somewhat above that in the conversion chamber bypassing it through the heat exchanger K and still B on its wav to thesaid chamber, and the other in which the valves a and 7a are closed andthe fresh oil is pumped through the valves 0 and c to meet the refluxedoil (as hereinafter described) and enters the conversion chamber at arelatively low temperature. In the latter case the necessary heat forthe conversion chamber is obtained by circulating the oil from itthrough the still B by means of the pump 0, the oil being heated in thestill to a temperature above that in the conversion chamber itself, thefresh and refluxed oil maintaining the chamber at the correcttemperature. The heat exchanger K is not employed in this method ofsupply. The general process is however the same in either method ofsupplying the fresh oil.

The refluxing chamber D into which the vapours and gases pass from theconversion chamber C is connected through a relief valve (I, set tooperate at the maximum pressure desired for cracking, to the expansionchamber G into which the light cracked oils and gases pass through thesaid valve, this chamber being maintained at a relatively lowtemperature and connected by the tube 7 to the dephlegmator orrectification column F. A portion of the vapours passing into thechamber D is condensed and returned to the conversion chamber C throughthe cooler E, so as to drop upon the upper surface of the oils in thechamber C. The heavy oil from the chamber C leaves through the valve 0The heavier portions of the light oils carried over in vapour form fromthe refluxing chamber D are condensed both in the expansion chamber Gand in the column F and the condensed oils in the expansion chamber andin the lower part of the column are carried through pipes and f to thefuel oil receivers J, the hot oil from the rectifying column passingfirst through the heat exchanger K, around the coil, while the fuel fromthe expansion chamber passes through the residue cooler H. The fuel orheavy oils so removed can be taken for further cracking treatment. Theoils in the conversion chamber C may pass in part through the reliefvalve 1; and circulating pump 0" back to the inlet tube 1) of the pipestill" for heat circulating purposes, while part may pass through thesecond relief valve 0 to the feed end of the plant behind the pressurepump a only if it is desired to mix a portion of partly cracked oil withthe fresh oil for further treatment. The gas and light oils arewithdrawn from the top of the rectifying column F, which is providedwith a water cooling coil 7 at the upper end, through the outlet pipe fand valve f to the vapour refining plant L. thence through the pipe Zand valve Z to the main condenser M from which the light oils pass tothe gas separator N, the gases from which pass through the relief valven set at a pressure say of about 50 lbs. per square inch, to the gasholder P or in part to the pipe still for heating purposes. The refiningoperation carried out in the plant L may be of known type such as thepassing of the oils over bauxite, fullers earth, or like filteringmaterial. The light oils from the condenser M after passing through thegas separator are received by the receivers O The vapours from theconversion chamber C pass directly into the refluxing chamber D situatedimmediately above the chamber C and as this refluxing chamber D ismaintained at any desired temperature below that of the conversion orcracking chamber light oils whose boiling point is below any givenlevel. for example 200 C., pass away as vapour to the expansion chamber,while oils whose boiling point is above this level pass first into thereflux cooler E in which theyare cooled before re-cntering theconversion chamber with the fresh oil. The temperature in the refluxingchamber is critical for any given range of light oils. It is wellunderstood that as the pressure on an oil mixture rises the boilingpoint rises rapidly but the boiling point of the lighter portions risesmore rapidly than that of the heavier portions so that the differencebetween the boiling points is reduced by the pressure. The refluxingtemperature therefore must be maintained constant within narrow limits.

The oil from the conversion chamber C is taken through the pipe 0 andrelief valve 0 I so removed passing through the coil f in the lower endof therect-ification column F so as to heat the lower end of this columnto any desired temperature, the oils then passing through the ex ansionvalve 9 to the expansion chamber Jr which is provided with a reliefvalve The oils taken direct from the conversion chamber into theexpansion chamber in this manner contain both light and heavyingredients which are separated in the expansion chamber and in therectification column to which the vapours are led from the expansionchamber as already mentioned.

The temperatures and pressure employed a temperature of 450 to 550 C.with a pressure of about 600 to 1,000 lbs, per square inch would beemployed. The fresh oil would be preheated, if supplied through thestill B, to a temperature between 500. and 600 C. to maintain theconversion chamber at the above temperature, while if admitted directlyto the conversion chamber no preheating is required and the circulationthrough the pump 0 and the still is regulated to maintain the conversionchamber temperature between 450 and 550 (1., allowing for the admissionof the-cold oil. The pump supplying the fresh oil would operate at about600 to 1,000 lbs. per square inch, corresponding to the pressure in theconversion chamber. The refluxing chamber D into which the vapours ofthe cracked oils pass would be heated to a temperature above 200 C. andusually to say about 360 to 400 C. and its pressure, of course,would bethat of the conversion chamber with which it is in communication,preferably through apertures in a tubular extension of the latterchamber projecting through the bottom of the refluxing chamber towardsthe upper end, the reflux cooler E forthe returned oils being connectedto the bottom of the cham- I her is allowed to collect in this chamberand -chamber G is connected to the column F.

through the valves 9 set to open at about 75 lbs. The outlet for the gasand spirit from the column F, is valve controlled by the relief valveopening at about 25 lbs. pressure.

The condensed oil in the expansion champasses to the outlet, which maylead to the heavy oil tank, through a relief valve'set at about 75 lbs.The coil f connected to the conversion chamber and placed in the lowerpart, of the rectification columir is connected through the relief valve9 (set to open at about the pressure in the conversion (chamber) to apipe opening into the expansion chamber G some or internally, by burningoil or gas produced in the cracking process or other suitable fuel, byelectrical means, or by superheated steam or vapour. Or as describedabove with reference to the diagram the preheating of the oil itself may'be suflicient to maintain the temperature required in the conversionchamber, which is in this case protected against loss of heat.

The temperature of the refluxing chamher is adjusted to a point which isbelow the critical temperature corresponding with the pressure for thelightest 011 which it is not desired to remove in a state of vapour, sothat the lower the maximum boiling point required for the cracked oilthe lower will be the temperature of the refluxing sys treatment of aparticular oil (Baku Mazout of specific gravity 0.901) as carried outTemperature of reaction or conversion chamber 525 C. under a pressure of800 lbs. Temperature of the refluxing chamber 365 C. at, of course, thesame pressure.

The treatment resulted in a conversion to light spirit (specific gravity0.739 and boiling point up to 200 C.) of 47 per cent of the heavy oiltreated. The gas produced amounted to 13.3 per cent and a residue of 39percent was left having a specific gravity of 0.924;.

Whatwe claim and desire to secure by Letters Patent of the United States1s:-

1. A process for the production of light from heavy oils which consistsin heating the oils rapidly to the cracking temperature in a tube still,passing the heated oils into a conversion chamber in which they aremaintained in bulk at ,the crackng temperature and at a pressure "offrom600 to 1,000

lbs. per square inch, drawing oil the light r oils in vapour form into avapour pressure or refluxing chamber open directly to and forming anextension of the conversion chamber and consequently at the samepressure of 600 to 1,000 lbs. per square inch, maintaining the saidrefluxing chamber at a temperature substantially below that of theconversion chamber, but sufiiciently high to prevent, under the saidpressure common to the conversion and refluxing chambers, thecondensation of any oils below a predetermined density, condensing theheavier oils in the aforesaid refluxing chamber and then passing thecondensed oils without reheatin and below the cracking temperaturethrougfl a branch passage directly back into the conversion chamber forfurther treatment,

simultaneously drawing off liquid oils from the conversion chamber,releasing the said liquid oils through a pressure reducing valve into alower pressure chamber, and removing the vapour of the lighter oils fromthe said lower pressure chamber.

2. In a cracking process as in claim 1, passing the oils condensed inthe refluxing chamber, prior to their return to the conversion chamber,through a reflux cooler while maintaining them under the pressure in theconversion chamber.

3. In a cracking process as in claim 1, passing the hot liquid oilsremoved from the conversion chamber through a pipe coil in arectification chamber for the lighter oils before passing to the saidlower pressure chamber.

OWEN DAVID LUCAS. ERNEST LAWSON LOMAX.

